The present invention is directed to an apparatus for generating digital gating signals in response to a data signal. Specifically, the present invention is directed to a three-state apparatus for generating digital gating signals which is capable of generating a high signal, a low signal, and a HIGH-Z (OFF) signal.
According to generally accepted industry standards, high signals may be generated as a "0" on both N-channel and P-channel drive lines from a buffer driver to a buffer to result in a "1" signal at the driver output; a low signal at a buffer output is represented by a "1" signal at both the P-channel and N-channel drive lines from a buffer driver to a buffer to result in a "0" signal at a buffer output. A HIGH-Z signal (OFF) is represented by a "1" signal on a P-channel drive line from a buffer driver and a simultaneous "0" signal on an N-channel drive line from a buffer driver. Accordingly, it is necessary, in order to generate the accepted definition of a HIGH-Z (OFF) signal to isolate the N-channel and P-channel drive lines from a buffer driver. The present invention provides an apparatus configured to enable such isolation of N-channel and P-channel drive lines appropriate for providing a HIGH-Z (OFF) signal in response to an ENABLE signal. The present invention will isolate the N-channel and P-channel drive line outputs appropriately to define a HIGH-Z (OFF) signal when the ENABLE signal is in a first state and will electrically connect the N-channel and P-channel output drive lines when the ENABLE signal is in a second state.
A further improvement associated with the present invention is a DRIVE-HIGH feature which provides for a feedback sensing of the data signal output of a buffer driven by the apparatus of the present invention to sense when that driver signal goes high. When the driver signal goes high, the DRIVE-HIGH feature responds to the driver signal by electrically disconnecting the drive signal output lines and maintaining that electrical isolation until the state signal changes to a second state, regardless of any variance of the data signal received by the apparatus. Such a momentary DRIVE-HIGH feature provides for rapid response of the apparatus of the present invention when the apparatus is going to a HIGH-Z (OFF) configuration without requiring the P-channel drive signal line to ramp up (relatively slowly) to its positive value as required by the HIGH-Z (OFF) signal definition. Such relatively slow ramping up is inherent by the RC time circuit established by effective high capacitance of a driver circuit and associated resistances inherent within the driver circuit.